Methods for transmitting data in a mobile system and radio stations therefor

ABSTRACT

The present invention relates to a method for transmitting a plurality of data streams from a primary station to a secondary station on a plurality of transmission beams, said method comprising: at the secondary station measuring the channel quality corresponding to each transmission beam, and signalling at a first rate to the primary station in a first indicator a CQI report representative of the quality of the beams, and signalling at a second rate, greater than the first rate, in a second indicator a number of data streams that can be transmitted on the transmission beams; at the primary station transmitting the data streams on the basis of the first and second indicators.

This application claims the benefit or priority of and describesrelationships between the following applications: wherein thisapplication is a continuation of U.S. patent application Ser. No.12/438,030 filed Feb. 19, 2009, which is the National Stage ofInternational Application No. PCT/IB2007/053330 filed Aug. 21, 2007,which claims the priority of foreign application EP06119342.1 filed Aug.22, 2006, all of which are incorporated herein in whole by reference.

The present invention relates to a method for transmitting data in amobile telecommunication system, to a primary station and to a secondarystation associated.

This invention is, for example, relevant for the mobile networks likethe UMTS, and especially for radio systems with terminals having aplurality of antennas.

The following 3GPP (3rd Generation Partnership Project) documents areincorporated by reference. In particular, they contain details about theterminology used in the present patent application. The documents arepublicly available for download from www.3gpp.org.

-   -   a) 3GPP Technical Report 25.876 V1.8.0 (2005-10) “Multiple Input        Multiple Output (MIMO) antennas in UTRA”, particularly for        Double Transmit Antenna Array (D-TxAA).    -   b) 3GPP Technical Specification 25.212 “Multiplexing and channel        coding (FDD)”, particularly for HS-SCCH and DPCCH channel        formats.    -   c) 3GPP Technical Specification 25.214 “Physical Layer        Procedures (FDD)”, particularly for Feedback Information (FBI)        signalling from UE to BS comprising preferred antenna weights        for a BS to use in downlink for closed-loop beamforming. Also        for “closed loop mode 1” which is another name for the “TxAA”        closed loop beamforming scheme specified in UMTS.

In 3GPP a proposal called D-TxAA for Double Transmit Antenna Array, isunder discussion for UMTS as a way of increasing the peak bit rate. Thisis derived from an existing closed loop transmit diversity scheme (TxAAmode 1) where the mobile terminal signals to the network indicators ofcomplex weights which should be applied to the signals from each of twotransmit antennas. In D-TxAA, two different data streams are transmittedusing orthogonal weight vectors, one weight vector being based on thosetransmitted from the mobile terminal, and the other vector being deriveddeterministically from the first.

For the operation of D-TxAA, the following may be assumed:

-   -   Orthogonal pilot channels are transmitted from each Node B        antenna.    -   No dedicated (i.e. beamformed) pilots are available (assuming        that the fractional dedicated physical channel (F-DPCH) is used,        which does not carry pilot bits).    -   Feedback information (FBI) for the first beam is derived by the        UE and transmitted to Node B, indicating the desired beamforming        vector.    -   The first beam is transmitted using a restricted code book of        weight vectors (for example the codebook currently used for TxAA        mode 1).    -   The identity of the antenna weight vector for the first beam is        signalled to the UE on the High-Speed Shared Control Channel        (HS-SCCH).    -   The second beam is transmitted using a deterministic phase        vector which is typically orthonormal to the vector for the        first beam.    -   Channel quality information (CQI) is signalled periodically by        the UE to the Node B, enabling the Node B to derive a different        rate for each of the two beams.    -   The CQI indicates the rate (or packet size) which can be        transmitted successfully (or with a given probability of        success) using a reference power level and code resource (the        reference values being known by both network and mobile        terminal)    -   The transmissions on the two beams are comprised of separate        codewords with potentially different rates.

In order for the Node B to determine how many codewords can besimultaneously transmitted, it needs information about how many andwhich beams can support the transmission of a codeword. Typically thisis done by CQI reporting, which also includes information about thesupportable rate for each codeword. However, frequent CQI reportingresults in a high signalling load. A reduced CQI reporting rate may beused, but the Node B will then be slower to react to changes in the rankof the channel, resulting in failed transmissions of codewords on beams,which cannot support transmission, or wasted capacity when beams are notused which could support a transmission.

It is an object of the invention to propose a method of signallingquickly available resources without causing too much overhead.

It is another object of the invention to propose a method enabling touse all the available resources at any time.

To this end, according to the invention, it is proposed a method fortransmitting data from a primary station to a secondary station, on aplurality of data streams, said method comprising:

at the secondary station measuring the quality of the data streams, andsignalling at a first rate to the primary station in a first indicator aCQI report representative of the quality of the data streams, andsignalling at a second rate, greater than the first rate, in a secondindicator a quantity of data that can be transmitted on the datastreams;

at the primary station transmitting the data streams on the basis of thefirst and second indicators.

According to the invention, a fast rank indicator is signalled from thesecondary station or User Equipment (UE) to the primary station or NodeB to enable rapid adjustment of the number of simultaneously-transmittedcodewords without requiring CQI to be transmitted every sub-frame. Theindicator may only indicate the number of available data streams thatcan support a codeword. Thus, the size of this indicator is reduced and,even if this indicator is sent very frequently (in each frame orsubframe), it does not lead to too much overhead.

As a consequence, this second indicator permits to have a flexiblecontrol of the number of codewords allocated to the set of datastreams,preventing thus the wasting of capacity, or problems of transmissions.

The present invention also relates to a method for transmitting aplurality of data streams from a primary station to a secondary stationon a plurality of transmission beams, said method comprising:

at the secondary station measuring the channel quality corresponding toeach transmission beam, and signalling to the primary station aplurality of antenna weights;

at the primary station computing at least a first transmission beam onthe basis of the plurality of antenna weights, and

transmitting the data streams on the transmission beams including thefirst transmission beam.

These and other aspects of the invention will be apparent from and willbe elucidated with reference to the embodiments described hereinafter.

The present invention will now be described in more detail, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of a system comprising a primary station and asecondary station in accordance with the invention;

FIG. 2 is a time chart representing the channel format in accordancewith an embodiment of the invention.

The present invention relates to a system of communication 300 asdepicted in FIG. 1, comprising a primary station 100, like a basestation, and at least one secondary station 200 like a mobile station.More specifically, this invention can be applied in multi-antennacommunication systems, and in particular, potential application isforeseen in the MIMO feature currently being standardised for UMTSRelease 7.

The radio system 300 may comprise a plurality of the primary stations100 and/or a plurality of secondary stations 200. The primary station100 comprises a transmitter means 110 and a receiving means 120. Anoutput of the transmitter means 110 and an input of the receiving means120 are coupled to an antenna array 130, including at least twoantennas, by a coupling means 140, which may be for example a circulatoror a changeover switch. Coupled to the transmitter means 110 andreceiving means 120 is a control means 150, which may be for example aprocessor. The secondary station 200 comprises a transmitter means 210and a receiving means 220. An output of the transmitter means 210 and aninput of the receiving means 220 are coupled to an antenna 230, or to anantenna array, by a coupling means 240, which may be for example acirculator or a changeover switch. Coupled to the transmitter means 210and receiving means 220 is a control means 250, which may be for examplea processor. Transmission from the primary radio station 100 to thesecondary station 200 takes place on a first channel 160 andtransmission from the secondary radio station 200 to the first radiostation 100 takes place on a second channel 260.

The first channel 160 takes place on a plurality of transmission beams.These transmission beams may be steered to be directed along at leastone direction preferred by the secondary station 200. To signal thequality of the transmission beams, CQI reports are transmittedperiodically. Typically, such CQI reports may indicate the quality ofthe beams, or a rate (or packet size) which can be transmittedsuccessfully for each data streams.

According to the invention, a fast rank indicator is signalled from thesecondary station 200 to primary station 100 to enable rapid adjustmentof the number of simultaneously-transmitted codewords or data streamswithout requiring CQI to be transmitted every sub-frame. This fast rankindicator is coded with a low number of bits, so that the frequenttransmission of this indicator does not lead to too much overhead. Asillustrated on FIG. 2, this indicator may be transmitted every subframe.On the other hand, the CQI reports may not be transmitted so often, andcould be transmitted for instance every four subframes, or even lessoften, so that it reduces the overhead.

In an embodiment, the fast rank indicator is time-multiplexed withantenna weight feedback, by observing that the relationship between thesize of the codebook (and therefore the number of bits required forantenna weight feedback) and the length of the subframe (and thereforethe number of physical channel bits available for transmitting antennaweight feedback) may result in one or more spare bit-positions which canbe used for transmitting the fast rank indicator.

Indeed, In RAN1#45 and RAN#32, it was agreed that the MIMO scheme forRe1-7 would be a dual-codeword MIMO scheme based on D-TxAA, with theantenna weights being signalled on the HS-SCCH.

Hereafter, the term “primary beam” will be used to designate the beamthat is formed on the basis of the secondary station 200 feedback. Thebeam that is formed using an orthogonal weight vector to that of thefirst beam is referred to as the “secondary beam”. However, it ispossible that this secondary beam is formed with help on a secondfeedback from the secondary station or for instance on a spatialmeasurement of the noise.

With D-TxAA, the primary beam is configured according to the feedbackfrom the UE in a similar way to the beamforming for closed-loop mode 1.A criterion for D-TxAA feedback may be chosen, so that the criterion forcalculating the FBI feedback bits would allow the possibility ofmaximising the received SIR. This would allow the secondary station 200with more than one receive antenna the possibility of taking intoaccount the spatial characteristics of any strong interferer(s). In thecase of spatially-white interference, this change would make nodifference, but it would be especially useful in the case when thestrongest eigenvector of the channel from the serving Node B happens tocoincide with a strong path from an interferer. However, in a variant ofthe invention, it is possible to choose the feedback so that the“received power” is maximised. In order to allow the secondary stationsto reuse existing algorithms, this change could be optional—i.e.allowing one of two behaviours for the secondary station (eithermaximising power or maximising SIR).

Concerning the signalling of feedback bits, in order to keep as muchcommonality as possible with the existing closed loop mode 1, it seemsreasonable to use the DPCCH FBI bits for the secondary station 200 totransmit its preferred antenna weight information to the primary station100. The fact that these bits are uncoded and may have a high error rateshould have less impact than for closed loop mode 1, as the signallingof the antenna weights on the HS-SCCH should be more reliable than anyantenna verification scheme implemented at the secondary station.

Some changes are, however, necessary compared to closed loop mode 1.Firstly, as the antenna weights will be signalled on the HS-SCCH in thedownlink, it can be advantageous that the antenna weights in thedownlink are fixed for the duration of an HS-DSCH subframe. This wouldalso aid channel estimation and decoding in the secondary station.

This means that 3 UpLink DPCCH FBI bits are available per HS-DSCH TTI(or 6 depending on the DPCCH slot format).

It is necessary to consider the exact usage of these FBI bits, and alsothe calculation of the phase vector at the Node B. In closed loop mode1, an averaging operation is employed to calculate the actualbeamforming weight vector to apply at the Node B based on the FBImessages received in two consecutive slots.

A number of possibilities exist for the exact way in which the FBI bitsare used and the primary beam phase vector calculated. For example:

-   -   1. Keep the definition of the FBI message exactly as in closed        loop mode 1, with a π/2 rotation per slot and require that Node        B calculates the primary beam antenna weights as an average of 2        slots. The code book used by the Node B would remain exactly as        in closed loop mode 1. As the HS-DSCH subframe is 3 slots, this        means that the FBI bit(s) in 1 slot out of every 3 would serve        no purpose. Therefore this bit is available for uplink        signalling. For example, it could be used as a fast “rank        indicator”, to indicate to the Node B whether the second beam is        able to support a second codeword or not.    -   2. Keep the definition of the FBI message exactly as in closed        loop mode 1, with a π/2 rotation per slot, but require that the        Node B calculates the primary beam antenna weights as an average        of 3 slots. This would have the effect of doubling the size of        the Node B code book, and adding 1 more bit to the HS-SCCH        signalling.

Indeed, in a variant of the invention, the secondary station may signalto the primary station the complex weights for building the primarybeam, with help of a plurality of antenna weights. The primary stationconstructs the primary beam by deducing it from the plurality ofweights, for instance by averaging the complex weights.

For example, the primary station is able to compute the primary byderiving an average from 3 antenna weights, doubling thus the size ofthe primary station codebook, and adding one bit available for thesignaling.

-   -   3. Keep the definition of the FBI message exactly as in closed        loop mode 1, with a π/2 rotation per slot, but leave the Node B        free to use any method to calculate the primary beam antenna        weights. The codebook of weights used for beamforming could be        the same as the current codebook for closed loop mode 1, or be        extended by additional entries, or be a different codebook. This        may be incompatible with some Node B architectures.    -   4. Redefine the usage of the FBI bits, and use repetition across        the 3 available bits per subframe. This would minimise the        number of FBI bits required per slot, and gain maximum advantage        from time-diversity. However, the time delay between the UE's        CPICH phase measurement and the application of the antenna        weights on HS-DSCH would increase. Furthermore, restricting the        beam rotation to ±π/2 between each HS-DSCH subframe would be        unlikely to fit the channel conditions.    -   5. Redefine the usage of the FBI bits, and use a simple code to        allow the UE to request any of the 4 possible phase vectors.        These could, for example, be coded as 0 0 0, 0 1 1, 1 1 0, 1 0        1, with a distance of 2 bits between each codeword. This        overcomes the restriction of the ±π/2 rotation for option 4, but        still has the disadvantage of the higher time delay than options        1 or 2.

This is illustrated in the timing diagram of FIG. 2.

The criterion for calculating the FBI feedback bits should allow thepossibility of maximising the received SIR, not the “received power” ascurrently specified in TS25.214 for closed loop mode 1.

The antenna weights in the downlink are fixed for the duration of anHS-DSCH subframe.

The basic definition of the FBI bits, averaging at the Node B, and thecodebook may be kept as in closed loop mode 1, and the redundant thirdFBI field is used as a fast rank indicator.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the art of radio communicationand the art of transmitter power control and which may be used insteadof or in addition to features already described herein.

The invention claimed is:
 1. A secondary station comprising: a receiverconfigured to simultaneously receive a plurality of data streams from aprimary station on a respective plurality of transmission beams, asensor configured to detect indications of channel quality of thetransmission beams, and processing circuitry configured to computeantenna weight feedback information based on channel quality of eachtransmission beam, and a transmitter configured to signal at a firstrate to the primary station in a first indicator a Channel QualityInformation (CQI) report representative of the quality of the pluralityof transmission beams, and to signal at a second rate to the primarystation, greater than the first rate, in a second indicator a number ofdata streams that can be transmitted on the transmission beams, whereinthe second indicator is multiplexed with the antenna weight feedbackinformation.
 2. The secondary station of claim 1, wherein the secondindicator indicates which transmission beams among the plurality oftransmission beams can support a data stream.
 3. The secondary stationof claim 1, wherein the transmission beams are transmitted usingorthogonal weight vectors.
 4. The secondary station of claim 1, whereinthe first rate equals zero.
 5. The secondary station of claim 1, whereinthe transmitter is configured to transmit to the primary stationcharacteristics of a first transmission beam.
 6. The secondary stationof claim 5, wherein the transmission of the characteristics of the firsttransmission beam comprises transmitting two indications of antennaweights included in two respective slots, and wherein the secondindicator is coded in a third slot.
 7. The secondary station of claim 1,wherein the second indicator indicates a number of available datastreams that can support a code word, whereby a number of simultaneouslytransmitted codewords can be rapidly adjusted without requiring channelquality information to be transmitted every subframe, and whereby aflexible control of a number of codewords allocated to the plurality ofdata streams is permitted.
 8. The secondary station of claim 1, whereinthe sensor detects an indication of quality of each respectivetransmission beam and the CQI report represents the quality of eachrespective transmission beam.
 9. A primary station comprising atransmitter configured to simultaneously transmit a plurality of datastreams to a secondary station on a respective plurality of transmissionbeams, a receiver configured to receive a first indicator from thesecondary station, sent at a first rate, the first indicator including aChannel Quality Information (CQI) report representative of the qualityof the plurality of transmission beams, and to receive a secondIndicator from the secondary station, sent at a second rate, greaterthan the first rate, the second indicator being representative of anumber of data streams that can be simultaneously transmitted on thetransmission beams, the first and second indicators being sent atperiodic or intermittent times; and wherein the receiver is configuredto receive antenna feedback information that is multiplexed with thesecond indicator, the antenna weight feedback information having beencomputed by the secondary station based on channel quality of eachtransmission beam of the plurality of transmission beams.
 10. Theprimary station of claim 9, wherein the second indicator indicates whichtransmission beams among the plurality of transmission beams can supporta data stream.
 11. The primary station of claim 9, wherein thetransmission beams are transmitted using orthogonal weight vectors. 12.The primary station of claim 9, wherein the receiver is configured toreceive at the primary station characteristics of a first transmissionbeam, and compute the characteristics of other transmission beams fromthe characteristics of the first transmission beam.
 13. The primarystation of claim 12, wherein receiving characteristics of the firsttransmission beam comprises receiving two indications of antenna weightsincluded in two respective slots, and wherein the second indicator iscoded in a third slot.
 14. A method for transmitting a plurality of datastreams from a primary station to a secondary station, the methodcomprising: simultaneously transmitting, by the primary station, aplurality of data streams to a secondary station on respective pluralityof transmission data beams, receiving at the secondary stationsimultaneously receiving the plurality of data streams from therespective plurality of transmission beams, sensing at the secondarystation measuring an indication of channel quality corresponding to eachtransmission beam, computing, at the secondary station, antenna weightfeedback information based on channel quality of each transmission beam,signaling, by the secondary station, at a first rate to the primarystation in a first indicator a Channel Quality information (CQI) reportrepresentative of the quality of the plurality of transmission beams,and signaling to the primary station at a second rate, greater than thefirst rate, in a second indicator a number of data streams that can betransmitted on the transmission beams, and receiving at the primarystation the first and second indicators, wherein the second indicator ismultiplexed with the antenna weight feedback information.
 15. A systemfor transmitting a plurality of data streams from a primary station to asecondary station, the system comprising: a primary station comprising atransmitter configured to simultaneously transmit a plurality of datastreams to a secondary station on respective transmission data beams,the secondary station comprising: a receiver of the secondary stationsimultaneously receiving the plurality of data streams, a sensorconfigured to detect an indication of channel quality corresponding toeach transmission beam, processing circuitry configured to computeantenna weight feedback information based on channel quality of eachtransmission beam, and a transmitter configured to signal at a firstrate to the primary station in a first indicator a Channel Qualityinformation (CQI) report representative of the quality of the beams, andto signal at a second rate, greater than the first rate, in a secondindicator a number of data streams that can be transmitted on thetransmission beams, wherein the second indicator is multiplexed with theantenna weight feedback information, the primary station comprising: areceiver of the primary station receiving the first and secondindicators, the transmitter of the primary station adjusting thetransmission of the data streams depending on the first and secondindicators.
 16. A method for a secondary station to receive a pluralityof data streams from a primary station, the method comprising: asimultaneously receiving a plurality of data streams from a primarystation on a respective plurality of transmission beams, detecting anindication of channel quality corresponding to each transmission beam,computing antenna weight feedback information based on channel qualityof each transmission beam, and signaling at a first rate to the primarystation in a first indicator a Channel Quality Information (CQI) reportrepresentative of the quality of the plurality of transmission beams,and signaling at a second rate to the primary station, greater than thefirst rate, in a second indicator a number of data streams that can betransmitted on the transmission beams, wherein the second indicator ismultiplexed with the antenna weight feedback information.
 17. A methodfor a primary station to transmit a plurality of data streams to asecondary station, the method comprising: a transmitter simultaneouslytransmitting a plurality of data streams to a secondary station on arespective plurality of transmission beams, a receiver receiving a firstindicator, sent at a first rate, the first indicator Including a ChannelQuality Information (CQI) report representative of the quality of theplurality of transmission beams, and a second indicator, sent at asecond rate, greater than the first rate, the second indicator beingrepresentative of a number of data streams that can be simultaneouslytransmitted on the transmission beams, the first and second indicatorsbeing received at periodic or intermittent times; and wherein thereceiver is configured to receive antenna feedback information that ismultiplexed with the second indicator, the antenna weight feedbackinformation having been computed by the secondary station based onchannel quality of each transmission beam of the plurality oftransmission beams.
 18. The secondary station of claim 1, wherein thesecondary station is based on a Universal Mobile TelecommunicationsSystem (UMTS) and the Channel Quality Information (CQI) report isindicative of the quality of the signals on a High Speed Downlink SharedChannel (HS-DSCH).
 19. The primary station of claim 9, wherein thesecondary station is based on a Universal Mobile TelecommunicationsSystem (UMTS) and the Channel Quality Information (CQI) report isindicative of the quality of the signals on a High Speed Downlink SharedChannel (HS-DSCH).
 20. The method of claim 14, wherein the secondarystation is based on a Universal Mobile Telecommunications System (UMTS)and the Channel Quality Information (CQI) report is indicative of thequality of the signals on a High Speed Downlink Shared Channel(HS-DSCH).
 21. The system of claim 15, wherein the secondary station isbased on a I Universal Mobile Telecommunications System (UMTS) and theChannel Quality Information (CQI) report is indicative of the quality ofthe signals on a High Speed Downlink Shared Channel (HS-DSCH).
 22. Themethod of cam 16, wherein the secondary station is based on a UniversalMobile Telecommunications System (UMTS) and the Channel Qualityinformation (CQI) report is indicative of the quality of the signals ona High Speed Downlink Shared Channel (HS-DSCH).
 23. The method of claim17, wherein the secondary station is based on a Universal MobileTelecommunications System (UMTS) and the Channel Quality information(CQI) report is indicative of the quality of the signals on a High SpeedDownlink Shared Channel (HS-DSCH).